Numerous radiometric analyzers of blood have been disclosed, most of which involve fairly complex apparatus designed to provide automated handling of blood samples and test elements. Such handling usually included metering of samples onto the elements, incubation and detection of the resulting radiometric changes. Examples of such analyzers are described in, e.g., U.S. Pat. No. 4,152,390, issued on May 1, 1979.
To obtain a productive through-put rate, it is desirable that such analyzers accommodate a backlog of the test elements. Usually the backlog is temporarily stored in the incubator during the development stage. Some analyzers feature rotating circular incubators which involve the following complexities: Unless complicated equipment is added to load the incubator without stopping it, such incubators must include motors, gearing and the like to stop the incubator for loading and unloading, and to restart it. So that test elements can be loaded in any position that becomes available in the incubator, each element's location at a specific "address" in the rotating incubator must be "remembered," entailing computer tracking of the element for proper retrieval of the element and reloading of the incubator. Since the load station is fixed relative to the rotating incubator, access to the incubator is limited to those instances when an empty incubator position appears at the load station. Finally, an incubator that rotates with respect to the remaining apparatus must provide temperature control through a coupling system that accommodates relative movement, e.g., electric slip rings. Although all these aspects of the rotating incubator are well within the skill of the art, they increase the cost and complexity of the incubator, and weigh against on-site analysis, e.g., at the physician's office.
A further difficulty in designing such incubators occurs because certain test chemistries produce gases which, if carried over to the following test samples, would cause erroneous readings. For example, test elements that quantify levels of BUN usually produce ammonia. Examples of such test elements are described in U.S. Pat. No. 4,066,403, issued on Jan. 3, 1978. Ammonia that is intended to create a color change only internally in its test element can escape, in some instances. In the worst case, this gas is carried as a contaminant to the next test element in an incubator. At best, escaping gas represents analyte that will not be detected, possibly resulting in a low reading. Prior to this invention, the problem of contamination from escaping gas has been dealt with through the use of test element covers and incubator materials that absorb little of the gas in question.